Structure and method of fabricating a transistor having a trench gate

ABSTRACT

An integrated circuit transistor is fabricated with a trench gate having nonconductive sidewalls. The transistor is surrounded by an isolation trench filled with a nonconductive material. The sidewalls of the gate trench are formed of the nonconductive material and are substantially free of unetched substrate material. As a result, the sidewalls of the gate trench do not form an undesired conductive path between the source and the drain of the transistor, thereby advantageously reducing the amount of parasitic current that flows between the source and drain during operation.

This application is a divisional of U.S. patent application Ser. No.11/038,985, filed on Jan. 20, 2005, which is a divisional of U.S. patentapplication Ser. No. 10/712,212, filed on Nov. 13, 2003, now U.S. Pat.No. 6,949,795 issued Sep. 27, 2005 the entirety of which are herebyincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to integrated circuit transistors. Inparticular, the invention relates to a structure and method offabricating a transistor having a trench gate.

2. Description of the Related Art

Integrated circuit designers often desire to increase the density ofelements within an integrated circuit by reducing the size of theindividual elements and reducing the separation distance betweenneighboring elements. One example of a common integrated circuit elementis a transistor, which can be found in many devices, such as memorycircuits, processors, and the like. A typical integrated circuittransistor comprises a source, a drain, and a gate formed at the surfaceof the substrate.

Although it is generally desirable to reduce the size of integratedcircuit transistors, the ability to shrink certain dimensions, such asthe length of the gate, can be limited due to the voltage levels neededto perform certain operations. In one example, a relatively high amountof voltage can be required by some transistors used in flash memory toperform certain operations, such as program, read, and erase. Oneapproach for reducing the size of such transistors while maintaining thegate length necessary to satisfy the voltage requirements is to form thetransistor gate as a trench below the surface of the substrate.

In one example, a transistor gate can be implemented as a U-shapedtrench connecting the source and the drain. Such a U-shaped gate trenchmaintains the gate length while allowing the gate surface area todecrease.

SUMMARY OF THE INVENTION

Present fabrication methods for a transistor with a U-shaped gate trenchproduce a transistor structure that is prone to parasitic currenttraveling from the source to the drain along the gate trench sidewalldue to unetched substrate material present at the gate trench sidewall.In addition, it is difficult to maintain isolation between gate trenchbottoms of neighboring transistors using current fabrication techniques.A unique transistor structure and a fabrication process are disclosed toprevent unetched substrate material from forming along the gate trenchsidewall and to facilitate isolation of neighboring transistors.

In one embodiment of the present invention, a semiconductor devicecomprises a transistor having a source and a drain comprising asubstrate material and a gate trench between the source and the drain.The device also comprises an isolation trench filled with anonconductive material surrounding the transistor. The gate trench hassidewalls comprising the nonconductive material, which are substantiallyfree of the substrate material.

In another embodiment, an integrated circuit transistor comprises asource, a drain, and a gate trench between the source and the drain. Thegate trench has nonconductive sidewalls and has a first depth. Thetransistor is surrounded by an isolation trench having a second depththat is greater than the first depth. The nonconductive sidewalls of thegate trench are formed at a point toward the middle of the gate trenchand away from the isolation trench.

In another embodiment, a method of forming a semiconductor elementcomprises providing a semiconductor substrate having a hard mask layerdeposited thereon, patterning the hard mask layer with a first photomask, and etching the semiconductor substrate to form an isolationtrench having a first depth. The method further comprises patterning thehard mask layer with a second photo mask, and etching the semiconductorsubstrate to form a gate trench having a second depth and simultaneouslyetching the isolation trench to a third depth, wherein the third depthis greater than the second depth.

In another embodiment, a method of fabricating a transistor comprisespatterning a substrate with a first mask, forming an isolation trench inthe substrate, and depositing a nonconductive material in the isolationtrench. The method further comprises patterning the substrate with asecond mask and forming a gate trench surrounded by a ridge of substratematerial, wherein the ridge of substrate material is separated from asource, a drain, and a gate by a separation trench. The method furthercomprises filling the gate trench and the separation trench with thenonconductive material, patterning the nonconductive material with athird mask, and removing the nonconductive material from a region of thegate trench, thereby forming a trench with sidewalls comprising thenonconductive material.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the invention will now bedescribed with reference to the drawings of certain preferredembodiments, which are intended to illustrate, and not to limit, theinvention. Throughout the drawings, reference numbers are re-used toindicate correspondence between referenced elements.

FIGS. 1-10 illustrate the formation of an integrated circuit transistorin accordance with one embodiment of the present invention.

FIGS. 11-16 illustrate the formation of an integrated circuit transistorin accordance with another embodiment of the present invention.

More specifically, FIG. 1 is a perspective view of a semiconductordevice in which a transistor can be formed.

FIG. 2 illustrates an active area photo mask to be applied to the deviceillustrated in FIG. 1.

FIG. 3 illustrates the device of FIG. 1 after the active area photo maskof FIG. 2 has been applied to pattern the hard mask layer.

FIG. 4 illustrates the device of FIG. 3 after an isolation trench hasbeen partially etched.

FIG. 5 illustrates a gate area photo mask to be applied to the deviceillustrated in FIG. 4.

FIG. 6 illustrates the device of FIG. 4 after the gate area photo maskof FIG. 5 has been applied to further pattern the hard mask layer.

FIG. 7 illustrates the device of FIG. 6 after the gate trench and theisolation trench have been etched.

FIG. 8 illustrates the device of FIG. 7 after a nonconductive materialhas been applied as a blanket layer over the partially formedtransistor.

FIG. 9 illustrates a gate trench photo mask to be applied to the deviceillustrated in FIG. 8.

FIG. 10 illustrates the device of FIG. 8 after the gate trench photomask of FIG. 9 has been applied and a gate trench has been etched toform a transistor having a gate trench with nonconductive sidewalls.

FIG. 11 is a perspective view of another semiconductor device in whichan isolation trench has been formed and in which a transistor can beformed.

FIG. 12 illustrates a gate area photo mask to be applied to the deviceillustrated in FIG. 11.

FIG. 13 illustrates the device of FIG. 11 after application of the gatearea photo mask of FIG. 12 and subsequent etching.

FIG. 14 illustrates the device of FIG. 13 after a nonconductive materialhas been deposited over the partially formed transistor.

FIG. 15 illustrates a gate trench photo mask to be applied to the deviceillustrated in FIG. 14.

FIG. 16 illustrates the device of FIG. 14 after the gate trench photomask of FIG. 15 has been applied and a gate trench has been etched toform a transistor having a gate trench with nonconductive sidewalls.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

For purposes of illustration, various embodiments of the invention willbe described in the context of a transistor having a particularconfiguration. For example, in some embodiments, an integrated circuittransistor is formed in accordance with the present invention byperforming the method illustrated in FIGS. 1-10. In other embodiments,an integrated circuit transistor is formed in accordance with thepresent invention by performing the method illustrated in FIGS. 11-16.The details associated with these exemplary structures and methods areset forth to illustrate, and not to limit, the invention. The scope ofthe invention is limited only by the appended claims.

FIG. 1 is a perspective view of a semiconductor device 100 in which atransistor can be formed. The device 100 includes a substrate 110, whichmay comprise any of a wide variety of suitable materials. For example,while the substrate 110 illustrated in FIG. 1 comprises an intrinsicallydoped monocrystalline silicon wafer, those of ordinary skill in the artwill understand that the substrate 110 in other devices can compriseother materials and/or additional semiconductor layers.

Although a single device 100 is illustrated in FIG. 1, those of ordinaryskill in the art will understand that a plurality of semiconductordevices 100 are preferably fabricated simultaneously on the samesubstrate 110. For example, the device 100 can be one of many likedevices arranged in an array.

As illustrated in FIG. 1, the substrate 110 is coated with a layer ofmaterial 120 suitable to be used as a hard mask, in accordance with oneembodiment of the invention. The substrate 110 can also be coated withan optional nitride layer (not shown). The hard mask 120 can compriseTEOS, amorphous carbon, Si₃N₄, SiO₃N₄, SiC, or any other suitable hardmask material, having a thickness preferably within the range of about100 Å to about 700 Å, more preferably within the range of about 100 Å toabout 300 Å.

FIG. 2 illustrates an active area photo mask 130 to be applied to thedevice 100 illustrated in FIG. 1 to pattern the hard mask layer 120. Theshaded portion of the active area photo mask 130 represents the area inwhich the hard mask 120 will remain after applying conventionalphotolithography and etching techniques, and the unshaded portionrepresents the area in which the hard mask 120 will be removed.

FIG. 3 illustrates the device 100 of FIG. 1 after the active area photomask 130 has been applied, and the hard mask 120 has been patterned. Asillustrated in FIG. 3, the hard mask 120 remains over the area of thesubstrate 110 in which the transistor will be formed. The hard mask 120is removed, however, from an area 210 of the substrate 110 in which anisolation trench (also referred to as a shallow trench isolation, orSTI, trench) will be formed around the transistor. The hard mask 120 canbe patterned using conventional photolithography and etching techniquesthat are well known to those of ordinary skill in the art.

As illustrated in FIG. 4, an isolation trench 310 is etched into thearea 210 to a depth that is less than the desired final depth. In someembodiments, the isolation trench 310 is etched using a process such as,for example, ion milling, reactive ion etching, or chemical etching. Ifan etching process involving a chemical etchant is selected, any of avariety of well-known etchants can be used, such as for example, Cl₂.

FIG. 5 illustrates a gate area photo mask 320 to be applied to thedevice 100 illustrated in FIG. 4 to further pattern the hard mask 120.The shaded portion of the mask 320 represents the areas of the substrate110 and the hard mask 120 that will be protected during the subsequentetching step, and the unshaded portion of the mask 320 represents theareas that will be removed during the subsequent etching step.

The dashed line 330 represents the active areas of the partially formedtransistor (e.g., the source, drain, and gate), as defined by theremaining hard mask 120. The dashed line 330 does not form part of thegate area photo mask 320, and is illustrated primarily to show how themask 320 is aligned over the device 100. Because the purpose of the mask320 is only to remove the hard mask 120, the unshaded portion of themask 320 can advantageously be wider than necessary to allow forpossible mask misalignment. Thus, in some embodiments, the unshadedportion of the mask 320 extends into the area of the isolation trench310, as illustrated in FIG. 5.

FIG. 6 illustrates the device 100 of FIG. 4 after the gate area photomask 320 has been applied, and the hard mask 120 has been furtherpatterned. As illustrated in FIG. 6, the hard mask 120 remains over theareas of the substrate 110 in which the source 420 and drain 430 will beformed. The hard mask 120 is removed, however, from the area 410 of thesubstrate 110 in which the gate trench will be formed.

The hard mask 120 can be patterned using well-known photolithography andetching techniques. For example, in some embodiments, photoresist isdeposited as a blanket layer over the device 100 and exposed toradiation through the gate area photo mask 320. Following this exposure,the photoresist film is developed to form a photoresist mask on thesurface of the hard mask 120, and the hard mask 120 is etched to exposethe area 410 of the substrate 110 in which the gate trench will beformed using at least one suitable etching process. Suitable etchingprocesses, examples of which are described above, are well known tothose of skill in the art.

FIG. 7 illustrates the device 100 of FIG. 6 after the gate trench 510and the isolation trench 310 have been formed by etching the substrate110 with the hard mask 120 covering the source 420 and the drain 430,and after the hard mask material has been removed. In some embodiments,the gate trench 510 and the isolation trench 310 are etched using aprocess such as, for example, ion milling, reactive ion etching, orchemical etching. If an etching process involving a chemical etchant isselected, any of a variety of well-known etchants can be used, such asfor example, Cl₂. In some embodiments, the gate trench 510 forms withrounding at the bottom, or U-shaped, so as to avoid sharp bends in theinterface between the substrate 110 and the gate trench 510.

In a preferred embodiment, the sequence of etches illustrated in FIGS. 4and 7 can be timed such that the resulting depth of both the gate trench510 and the isolation trench 310 are as desired. The depth of theisolation trench 310 is preferably different than the depth of the gatetrench 510. For example, in some embodiments, the depth of the isolationtrench 310 is greater than the depth of the gate trench 510. In oneexemplary embodiment, the depth of the gate trench 510 falls within therange of about 50 nm to about 300 nm, preferably about 200 nm, and thedepth of the isolation trench 310 falls within the range of about 300 nmto about 500 nm, preferably about 350 nm.

As also illustrated in FIG. 7, the hard mask 120 covering the source 420and the drain 430 is removed using at least one suitable etchingprocess. Examples of suitable etching processes are described above andare well known to those of skill in the art.

FIG. 8 illustrates the device 100 of FIG. 7 after a nonconductivematerial 610 has been applied as a blanket layer over thepartially-formed transistor. The nonconductive material 610 can compriseany of a wide variety of materials, such as, for example, an oxidematerial, preferably a high density plasma oxide (HDP oxide). Thenonconductive material 610 can be deposited using any suitabledeposition process, such as, for example, chemical vapor deposition(CVD) or physical vapor deposition (PVD).

In some embodiments, the nonconductive material 610 fills the isolationtrench 310 and the gate trench 510. The nonconductive material 610 canthen be polished back to expose the source 420 and the drain 430 usingany suitable polishing process, such as, for example,chemical-mechanical planarization (CMP). Preferably, the hard maskmaterial (e.g. nitride) would remain over the source and drain and actas a CMP stop while protecting the source and drain. In this case, thehard mask would be removed after CMP, exposing the source and drainregions.

FIG. 9 illustrates a gate trench photo mask 620 to be applied to thedevice 100 illustrated in FIG. 8. The shaded portion of the mask 620represents the areas of the substrate 110 and the nonconductive material610 that will be protected during the subsequent etching step, and theunshaded portion of the mask 620 represents the areas that will beremoved during the subsequent etching step.

The dashed lines 630 represent the active areas of the partially formedtransistor (e.g., the source, drain, and gate). The dashed lines 630 donot form part of the gate trench photo mask 620, and are illustratedprimarily to show how the mask 620 is aligned over the device 100. Asdiscussed in more detail below, the unshaded portion of the mask 620 canadvantageously be larger than necessary to allow for possible maskmisalignment.

FIG. 10 illustrates the device 100 of FIG. 8 after the gate trench photomask 620 has been applied, and a gate trench 710 has been etched to forma transistor having a gate trench 710 with nonconductive sidewalls 720.The gate trench 710 can be formed using conventional photolithographyand etching techniques. Conventional photolithography and etchingtechniques are described above, and are well known to those of skill inthe art.

For example, in some embodiments, a photoresist film is deposited on thenonconductive material 610 and exposed to radiation through the gatetrench photo mask 620. Following this exposure, the photoresist film isdeveloped to form a photoresist mask on the surface of the nonconductivematerial 610, and the nonconductive material 610 is etched to form thegate trench 710. In some embodiments, the gate trench 710 is etchedusing a process such as, for example, ion milling, reactive ion etching,or chemical etching. If an etching process involving a chemical etchantis selected, any of a variety of well-known etchants can be used, suchas for example, CF₄.

Because the gate trench 710 is formed by etching the nonconductivematerial 610 rather than the substrate 110, a different etchant istypically used to perform this etch than the etchant used to form theisolation trench 310 and the gate trench 510, as described above inconnection with FIGS. 4 and 7. This is because very few, if any,etchants are used in conventional semiconductor processing that areeffective at etching both the nonconductive material 610 and thematerial comprising the substrate 110.

Due to the lack of such etchants, it has been difficult in the past tofabricate a transistor with a trench gate without also leavingundesirable sidewalls comprising unetched substrate material along thesides of the gate trench. Such sidewalls can form an undesiredconductive path through which parasitic current can flow between thesource and the drain of the transistor during operation.

Unlike a transistor having a trench gate formed using conventionalmethods, however, the transistor formed using the process illustrated inFIGS. 1-10 advantageously has a gate trench 710 with nonconductivesidewalls 720. By etching the gate trench 510 and completing the etch ofthe isolation trench 310 in the same step, the amount of unetchedsubstrate material along the sidewalls of the gate trench 510 is reducedor eliminated altogether. As a result, when the gate trench 710 isetched, the sidewalls 720 are formed of the nonconductive material 610rather than the substrate material. Because an undesired conductive pathis not formed at the intersection of the gate trench 710 and theisolation trench 310, the flow of parasitic current between the source420 and the drain 430 through the sidewalls 720 is advantageouslyreduced.

Another advantage of forming a transistor using the process illustratedin FIGS. 1-10 is that, in some embodiments, the isolation trench 310 canbe made deeper than the gate trench 710. By making the isolation trench310 deeper than the gate trench 710, the gate trench bottoms ofneighboring transistors can be advantageously isolated from one another.

Another advantage of forming a transistor using the process illustratedin FIGS. 1-10 is that the gate area photo mask 320 and the gate trenchphoto mask 620 can be made to allow for potential mask misalignment. Forexample, because the sidewalls 720 of the gate trench 710 comprise thenonconductive material 610 rather than the material of the substrate110, they do not form a conductive path between the source 420 and thedrain 430, and thus they can be offset from the outer edges of the gatetrench 710. Accordingly, in some embodiments, the unshaded portion ofthe gate trench photo mask 620 can be wider than necessary to allow forpossible mask misalignment. Because it can be difficult to preciselyalign multiple photo masks over the device 100 during successive maskingand etching steps, the allowance for potential mask misalignmentprovided by the process illustrated in FIGS. 1-10 presents significantadvantages.

Those of ordinary skill in the art will understand that a variety ofadditional processes can be performed in between any of theaforementioned processes. For example, conventional processes related tothe electrical characteristics of the transistor can be performed, suchas depositing doping implants or other additional layers. Suchadditional processes are well known to those of skill in the art.

FIG. 11 is a perspective view of a semiconductor device 800 in which atransistor can be formed. The device 800 comprises a semiconductorsubstrate 810 having an isolation trench 820 formed therein, which isfilled with a nonconductive material 830, as is well-known to those ofskill in the art. The substrate 810 of the device 800, like that of thedevice 100 illustrated in FIGS. 1-10, may comprise any of a wide varietyof suitable materials that are known to those of skill in the art. Inaddition, although a single device 800 is illustrated in FIG. 11, thoseof skill in the art will understand that a plurality of devices 800 arepreferably fabricated simultaneously on the same substrate 810.

FIG. 12 illustrates a gate area photo mask 840 to be applied to thedevice 800 illustrated in FIG. 11. The shaded portion of the mask 840represents the area of the substrate 810 that will not be etched afterapplying conventional photolithography and etching techniques, whereasthe unshaded portion of the mask 840 represents the area of thesubstrate 810 that will be etched. In the embodiment illustrated in FIG.12, the mask 840 extends into the area in which the isolation trench 820is formed.

The dashed line 850 represents the boundary between the upper surface ofthe substrate 810 and the surrounding isolation trench 830. The dashedline 850 does not form part of the gate area photo mask 840, and isillustrated primarily to show how the mask 840 is aligned over thedevice 800.

FIG. 13 illustrates the device 800 of FIG. 11 after application of thegate area photo mask 840 and subsequent etching. As illustrated in FIG.13, a gate trench 910 is etched into the substrate 810. In someembodiments, gate trench 910 forms with rounding at the bottom, orU-shaped, so as to avoid sharp bends in the interface between the gatetrench 910 and the substrate 810. In addition, an area of the substrate810 surrounding the source/gate/drain area 910, 920, 930 is etched toform a separation trench 940. The separation trench 940 is surrounded bya ridge 1140 of unetched substrate material. The source 920 and drain930 of the partially formed transistor also remain unetched.

The gate trench 910 and the separation trench 940 can be etched usingconventional photolithography and etching techniques. For example, insome embodiments, a photoresist film is deposited on semiconductordevice 800, exposed to radiation through the gate area photo mask 840,and developed to form a photoresist mask on the surface of the substrate810. The substrate 810 is then etched using at least one suitableetching process, such as, for example, ion milling, reactive ionetching, or chemical etching. If an etching process involving a chemicaletchant is selected, any of a variety of well-known etchants can beused, such as, for example, Cl₂.

FIG. 14 illustrates the device 800 of FIG. 13 after a nonconductivematerial 1010 has been deposited over the partially formed transistor.The nonconductive material 1010 fills the gate trench 910 and theseparation trench 940. The nonconductive material 1010 can comprise anyof a wide variety of materials, such as, for example, an oxide material,preferably a high density plasma oxide (HDP oxide). The nonconductivematerial 1010 can be deposited using any suitable deposition process,such as, for example, CVD or PVD. The nonconductive material 1010 canthen be polished back, if necessary, to expose the source 920 and thedrain 930 using any suitable polishing process, such as, for example,CMP.

FIG. 15 illustrates a gate trench photo mask 1020 to be applied to thedevice 800 illustrated in FIG. 14. The shaded portion of the mask 1020represents the areas of the substrate 810 and the nonconductive material820 that will be protected during the subsequent etching step, and theunshaded portion of the mask 1020 represents the areas that will beremoved during the subsequent etching step.

The dashed lines 1030 represent the active areas of the partially formedtransistor (e.g., the source, drain, and gate) and the surrounding ridge1140 of substrate material. The dashed lines 1030 do not form part ofthe gate trench photo mask 1020, and are illustrated primarily to showhow the mask 1020 is aligned over the device 800. As discussed above,the unshaded portion of the mask 1020 can advantageously be larger thannecessary to allow for possible mask misalignment.

FIG. 16 illustrates the device 800 of FIG. 14 after the gate trenchphoto mask 1020 has been applied and a gate trench 1110 has been etchedto form a transistor having a gate trench 1110 with nonconductivesidewalls 1120. The gate trench 1110 can be formed using conventionalphotolithography and etching techniques. Conventional photolithographyand etching techniques are described above, and are well known to thoseof skill in the art.

For example, in some embodiments, a photoresist film is deposited on thenonconductive material 1010 and exposed to radiation through the gatetrench photo mask 1020. Following this exposure, the photoresist film isdeveloped to form a photoresist mask on the surface of the nonconductivematerial 1010, and the nonconductive material 1010 is etched to form thegate trench 1110. In some embodiments, the gate trench 1110 is etchedusing a process such as, for example, ion milling, reactive ion etching,or chemical etching. If an etching process involving a chemical etchantis selected, any of a variety of well-known etchants can be used, suchas for example, CF₄.

As illustrated in FIG. 16, the transistor comprises a source 920, adrain 930, and a gate trench 1110 surrounded by a separation trench 940filled with a nonconductive material 1010. In some embodiments, thedepth of the separation trench 940 and the depth of the gate trench 1110are approximately equal and are shallower than the isolation trench 820.In one exemplary embodiment, the depth of the separation trench 940 andthe gate trench 1110 falls within the range of about 50 nm to about 300nm, preferably about 200 nm, and the depth of the isolation trench 820falls within the range of about 300 nm to about 500 nm, preferably about350 nm.

In the embodiment illustrated in FIG. 16, the separation trench 940surrounds the transistor on all sides. In other embodiments, theseparation trench 940 can have a different shape, such as one that doesnot completely surround the source 920 and the drain 930 of thetransistor. For example, in one embodiment, the shape of the separationtrench 940, when viewed from above, resembles a letter “H” (rather thana FIG. 8, as in the illustrated embodiment), such that one side of thesource 920 and one side of the drain 930 are in contact with the ridge1140 of substrate material.

Like the transistor discussed above, the transistor formed using theprocess illustrated in FIGS. 11-16 advantageously has a gate trench 1110with nonconductive sidewalls 1120. By etching the gate trench 910 andthe separation trench 940 in the same step, there is no unetchedsubstrate material along the sidewalls of the gate trench 910. As aresult, when the gate trench 1110 is etched, the sidewalls 1120 areformed of the nonconductive material 1010 rather than the substratematerial. Accordingly, an undesired conductive path is not formedbetween the source 920 and the drain 930, and the flow of parasiticcurrent through the sidewalls 1120 is advantageously reduced.

Another feature of forming a transistor using the process illustrated inFIGS. 11-16 is that the transistor is separated from the ridge 1140 ofsubstrate material by the isolation trench 820. Advantageously, thelocation of the ridge 1140 of substrate material avoids shorting betweenneighboring transistors.

In addition, as discussed above, an advantage of forming a transistorusing the process illustrated in FIGS. 11-16 is that gate trench photomask 1020 can be made to allow for potential mask misalignment. Becauseit can be difficult to precisely align multiple photo masks over thedevice 800 during successive masking and etching steps, the allowancefor potential misalignment of the gate trench photo mask 1020 presentssignificant advantages.

As discussed above, those of ordinary skill in the art will understandthat a variety of additional processes can be performed in between anyof the aforementioned processes. For example, conventional processesrelated to the electrical characteristics of the transistor can beperformed, such as depositing doping implants or other additionallayers. Such additional processes are well known to those of skill inthe art.

Although this invention has been described in terms of certain preferredembodiments, other embodiments that are apparent to those of ordinaryskill in the art, including embodiments that do not provide all of thefeatures and advantages set forth herein, are also within the scope ofthis invention. Accordingly, the scope of the present invention isdefined only by reference to the appended claims.

1. A method of fabricating a transistor comprising; patterning asubstrate with a first mask; forming a first isolation trench in thesubstrate; depositing a nonconductive material in the first isolationtrench; patterning the substrate with a second mask; forming a firstgate trench; forming a separation trench such that a ridge of substratematerial extends around at least portions of a source region, a drainregion and a gate region and wherein the separation trench is interposedbetween the ridge of substrate material and the source, drain, and gateregions along at least portions of the source, drain, and gate regions;filling the first gate trench and the separation trench with thenonconductive material; patterning the nonconductive material with athird mask; and removing the nonconductive material from at least aregion of the first gate trench, thereby forming a second gate trenchwith sidewalls comprising the nonconductive material.
 2. The method ofclaim 1, wherein the second mask extends into the area in which theisolation trench is formed.
 3. The method of claim 1, wherein the thirdmask contains at least one feature that is sized to allow for possiblemask misalignment.
 4. The method of claim 1, wherein the first gatetrench and the separation trench are approximately of equal depth. 5.The method of claim 1, wherein at least one of the second gate trenchand the separation trench are of a shallower depth than a depth of theisolation trench.
 6. The method of claim 1, wherein the separationtrench is not interposed between outer sides of the drain and source andthe ridge of substrate material.
 7. The method of claim 6, wherein thesecond gate trench, the isolation trench, and the sidewalls define incombination a generally H shaped structure.
 8. The method of claim 1,wherein the second gate trench, the isolation trench, and the sidewallsdefine in combination a generally FIG. 8 shaped structure.
 9. The methodof claim 1, wherein the nonconductive material comprises high densityplasma (HDP) oxide.
 10. The method of claim 1, wherein forming the firstgate trench and forming the separation trench are performed as the sameprocess step.
 11. The method of claim 1, wherein at least one of theisolation trench, the gate trench, and the separation trench is formedvia etching.
 12. A method of forming a semiconductor device, the methodcomprising: at least partially forming an isolation trench and a gatetrench in a substrate; forming a nonconductive material in the isolationtrench and the gate trench; and removing a least a portion of thenonconductive material in the gate trench so as to form sidewalls of thegate trench wherein the sidewalls comprise the nonconductive material.13. The method of claim 12, wherein the gate trench is formed to have arounded profile.
 14. The method of claim 12, further comprising forminga separation trench that extends around at least portions of a sourceregion, a drain region and a gate region.
 15. The method of claim 14,wherein the gate trench and the separation trench are formed to be ofapproximately equal depth.
 16. The method of claim 14, wherein the gatetrench and the separation trench are formed in the same process step.17. The method of claim 12, wherein the isolation trench is formed toextend to a deeper depth than a depth of the gate trench.
 18. The methodof claim 12, further comprising forming a drain and a source such thatthe drain trench is interposed between the source and the drain andwherein walls of the drain trench adjacent the respective source anddrain are tapered.
 19. The method of claim 12, wherein the gate trenchis interposed between a source and a drain and wherein the gate trenchis formed via forming a mask and etching the substrate through at leastone opening in the mask.
 20. The method of claim 19, wherein at leastone opening of the mask is formed to extend beyond a boundary betweenthe source and drain and the isolation trench in at least one dimension.21. A method of forming a semiconductor device, the method comprising:at least partially forming an isolation trench and a gate trench in asubstrate; forming a nonconductive material in the isolation trench andthe gate trench; and removing a least a portion of the nonconductivematerial in the gate trench so as to form sidewalls of the gate trenchwherein the sidewalls comprise the nonconductive material and such thatthe isolation trench had a greater depth than a depth of the gatetrench.
 22. The method of claim 21, wherein the gate trench isinterposed between a source and a drain and wherein the gate trench isformed via masking and etching the substrate.
 23. The method of claim22, wherein an opening of the mask is formed to extend beyond a boundarybetween the source and drain and the isolation trench in at least onedimension.
 24. The method of claim 21, further comprising forming aseparation trench that extends around at least portions of a sourceregion, a drain region and a gate region.
 25. The method of claim 24,wherein the gate trench and the separation trench are formed to be ofapproximately equal depth.
 26. The method of claim 24, wherein the gatetrench and the separation trench are formed in the same process step.27. The method of claim 24, wherein the gate trench and the separationtrench in combination define generally a FIG. 8 shape.